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Why is it desirable to correlate runway names with the Earth's magnetic fields? Is it because the navigational equipment breaks / becomes inaccurate when there

Why is it desirable to correlate runway names with the Earth's magnetic fields? Is it because the navigational equipment breaks / becomes inaccurate when there's a field shift? It seems like it could be confusing over time to keep updating runway names as flight paths are repeatedly flown by generally the same sets of pilots, and as paths becomes familiar. For fancy autopilot setups most of the in-the-moment thinking is avoided and this may become a very minor detail. Edit: Thanks for the quick info everyone! Now this makes sense. Happy holidays :)

SOTGO 4y ago
It's so that in the event of mechanical failure a pilot can always rely on a magnetic compass for navigation, without having to know or look up the magnetic variation of the airport.
- TylerE 4y ago
  And also so when they get cleared for “runway 15” they know which side to come on without having to consult a chart. Also so there’s no confusion, each direction is named, so a runway that runs due North-South will be Runway 36 in the northern direction, and 18 to the south. In practice it is rare for all but the largest airports to have more than 3 or 4 runways so it isn’t like there will be a Runway 2 and a Runway 3 at the same airport. If they need more capacity they’ll build parallel runways which will be named with a suffix, like 9L and 9R. (Also, magnetic compasses aren’t that accurate in the first place, since there’s iron in the engines if nothing else. In theory there should be a compensation/calibration done with the results on a card, but, no guarantees that it’s done or up to date).
  - pdonis 4y ago
    > If they need more capacity they’ll build parallel runways which will be named with a suffix, like 9L and 9R. There are even some airports that have three runways with the same heading, so they have "L", "C", and "R". (Two examples that I've personally took off or landed on all three of are 17/35 at DFW and 01/19 at IAD.)
    - sokoloff 4y ago
      Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, 3R, and 3L. https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF
      - asciimike 4y ago
        KDEN also has four parallels (16-34L/R and 17-35L/R). All are 172.6º or 352.6º.
        stevehawk 4y ago
        KDEN also put their runways so close together that it's not uncommon to for aircraft on approach on the parallel to set off another airplane's TCAS. I think most airline pilots consider that airport ripe for a midair.
      - Mountain_Skies 4y ago
        Atlanta has five parallel runways: 27R, 27L, 9R, 9L, and 28/10. It appears the main four that are close to each other are set alternating east landing and west landing as they're not marked on both ends like 28/10 is.
        sokoloff 4y ago
        They are numbered on both ends on both the satellite view (real-world) and on the FAA diagram: http://aeronav.faa.gov/d-tpp/2213/00026AD.PDF http://aeronav.faa.gov/d-tpp/2213/00026AD.PDF There would be no (or extremely rare, at a minimum) operational reason to alternate directions for takeoff and landings on parallel runways. (There are rules for simultaneous operations to parallel runways and having them be in opposite directions would be problematic as you have to assume some aircraft will execute a missed approach and would then be climbing into opposite direction landing traffic with only a small lateral offset.)
        TylerE 4y ago
  - s1dev 4y ago
    The correction card is legally required equipment and there are a few maintenance items that trigger a requirement to update it
  - OliverJones 4y ago
    With respect, calibrating compass compensations is in fact very important in FAA approved aircraft. It's part of the inspection checklist signed off by the licensed mechanic. My flight instructor once switched off the electricity to the airplane's controls and radios and told me to land. At an airport with a control tower. Using the magnetic compass, airspeed, barometric altimeter, RPMs, that's it. Nothing else works without electric power. There's a whole procedure for doing all that. Being able to navigate safely with an old-school magnetic compass is life-critical.
    - TylerE 4y ago
      Ok, that’s totally fair. It’s been 25 years since I took a few lessons, purely dim since then.
    - zzless 4y ago
      Not to doubt your story but being a flight instructor myself, I cannot imagine intentionally turning off power to airplane radios at an airport with a control tower (this is also illegal). As far as the pure mechanics of landing go, one does not need any instruments whatsoever: one can judge the airspeed by the sound and the wing angle. I routinely make my students do this when I teach tailwheel flying.
      - TheRealPomax 4y ago
        Not to mention that no one would ever be insane enough to turn off electronics and then say "right, now IFR your way in". You're flying VFR, you're going to have eyes on the runway.
      - stevehawk 4y ago
        > Not to doubt your story but being a flight instructor myself, I cannot imagine intentionally turning off power to airplane radios at an airport with a control tower (this is also illegal). You just pre-arrange it with the tower and it's no big deal. Probably exceptions for busy class Cs or Ds, but it's no big deal to any airport near me.
      - grepfru_it 4y ago
  - V99 4y ago
    There are only 3 suffixes, so if there are 4 or more parallel runways one will get the "wrong" number, like 2/3L/3C/3R.
Jalad 4y ago
I agree with the other commenter, it's also explained in this CGP Grey video https://youtu.be/qD6bPNZRRbQ https://youtu.be/qD6bPNZRRbQ
- mr_woozy 4y ago
  [dead]
Rebelgecko 4y ago
So that they line up with the magnetic compasses inside of the planes
nilsbunger 4y ago
It's a convention for A LOT of aspects of flying: air traffic control instructions ("turn left heading 090"), charts ("fly heading 070 after takeoff"), etc are all in magnetic coordinates. Planes also a normal compass as their ultimate backup instrument, which of course is also magnetic. Finally, while modern systems could operate off true north instead, not ALL planes have that.
inamberclad 4y ago
Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Also, winds (and just about everything else) are in magnetic degrees. If I have wind from 280 while I'm landing, then I immediately know that I have a left quartering headwind and what to expect.
- quest88 4y ago
  Haha of course someone on HN would use 31 at PAO as their example :).
  - fragmede 4y ago
    For the uninitiated, PAO is a small general aviation airport in the city of Palo Alto aka the heart of Silicon Valley. Nearby is also SQL, adjacent to Oracle's (former) HQ. That name is purely coincidence!
    - inamberclad 4y ago
      Hah, I never actually made the SQL <-> Oracle connection.
- deepsun 4y ago
  > Also, winds (and just about everything else) are in magnetic degrees. Not always: "If you read it, it's true. If you hear it, it's magnetic." https://aviation.stackexchange.com/a/62/51758 https://aviation.stackexchange.com/a/62/51758
- shostack 4y ago
  Having just flown out of there yesterday, on the return it is also helpful, it's also helpful to know without having to look it up what I should set my OBS to in order to help align for practicing my pattern and approach.
- DocTomoe 4y ago
  > Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Actually, it's more like "310ish". Both 305 and 314 are acceptable readings for a runway 31.
akira2501 4y ago
One other thing not covered, on any airport chart, the exact alignment of the runway will be noted, usually down a tenth of degree. So the numbering is a useful backup and discriminator, but the charts have much more detail, and modern navigational and instrument systems on runways obviate much of this for most flights anyways.
mmaunder 4y ago
It’s super helpful and intuitive when you’re flying. Took me a while to get the compass visualized in my head but now if I’m making a visual approach and they give me a runway I can look at my current magnetic heading on my heading indicator - which you periodically sync to your magnetic compass - and know exactly what direction relative to my current heading I’ll be landing. So that’s the fundamental reason why most things flight related are magnetic except rare items like winds aloft reports. Wind direction reports at the field? Magnetic, so that you know how many degrees off the wind is from your landing direction and can do a bit of mental math to get the crosswind component. If you’ve just taken off ATC may say fly runway heading - again an easy one using your heading indicator which is synced to your magnetic compass. As a side note, modern avionics like G1000 auto sync your heading indicator to magnetic. So if we started flying with electronic avionics that are auto synced and have a database of magnetic variation around the world, then it’s feasible to always show true on the heading indicator and have runways and everything else be true bearings. But that’s not the case and I still fly some of the many steam dial planes out there as do others, so we’re stuck with magnetic runways and magnetic everything else used in real-time flight and situational awareness for pilots.

SOTGO 4y ago

It's so that in the event of mechanical failure a pilot can always rely on a magnetic compass for navigation, without having to know or look up the magnetic variation of the airport.

TylerE 4y ago
And also so when they get cleared for “runway 15” they know which side to come on without having to consult a chart. Also so there’s no confusion, each direction is named, so a runway that runs due North-South will be Runway 36 in the northern direction, and 18 to the south. In practice it is rare for all but the largest airports to have more than 3 or 4 runways so it isn’t like there will be a Runway 2 and a Runway 3 at the same airport. If they need more capacity they’ll build parallel runways which will be named with a suffix, like 9L and 9R. (Also, magnetic compasses aren’t that accurate in the first place, since there’s iron in the engines if nothing else. In theory there should be a compensation/calibration done with the results on a card, but, no guarantees that it’s done or up to date).
- pdonis 4y ago
  > If they need more capacity they’ll build parallel runways which will be named with a suffix, like 9L and 9R. There are even some airports that have three runways with the same heading, so they have "L", "C", and "R". (Two examples that I've personally took off or landed on all three of are 17/35 at DFW and 01/19 at IAD.)
  - sokoloff 4y ago
    Or 4 parallel runways like Detroit Metro (KDTW): 4R, 4L, 3R, and 3L. https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF https://aeronav.faa.gov/d-tpp/2213/00119AD.PDF
    - asciimike 4y ago
      KDEN also has four parallels (16-34L/R and 17-35L/R). All are 172.6º or 352.6º.
      - stevehawk 4y ago
        KDEN also put their runways so close together that it's not uncommon to for aircraft on approach on the parallel to set off another airplane's TCAS. I think most airline pilots consider that airport ripe for a midair.
    - Mountain_Skies 4y ago
      Atlanta has five parallel runways: 27R, 27L, 9R, 9L, and 28/10. It appears the main four that are close to each other are set alternating east landing and west landing as they're not marked on both ends like 28/10 is.
      - sokoloff 4y ago
        They are numbered on both ends on both the satellite view (real-world) and on the FAA diagram: http://aeronav.faa.gov/d-tpp/2213/00026AD.PDF http://aeronav.faa.gov/d-tpp/2213/00026AD.PDF There would be no (or extremely rare, at a minimum) operational reason to alternate directions for takeoff and landings on parallel runways. (There are rules for simultaneous operations to parallel runways and having them be in opposite directions would be problematic as you have to assume some aircraft will execute a missed approach and would then be climbing into opposite direction landing traffic with only a small lateral offset.)
        TylerE 4y ago
- s1dev 4y ago
  The correction card is legally required equipment and there are a few maintenance items that trigger a requirement to update it
- OliverJones 4y ago
  With respect, calibrating compass compensations is in fact very important in FAA approved aircraft. It's part of the inspection checklist signed off by the licensed mechanic. My flight instructor once switched off the electricity to the airplane's controls and radios and told me to land. At an airport with a control tower. Using the magnetic compass, airspeed, barometric altimeter, RPMs, that's it. Nothing else works without electric power. There's a whole procedure for doing all that. Being able to navigate safely with an old-school magnetic compass is life-critical.
  - TylerE 4y ago
    Ok, that’s totally fair. It’s been 25 years since I took a few lessons, purely dim since then.
  - zzless 4y ago
    Not to doubt your story but being a flight instructor myself, I cannot imagine intentionally turning off power to airplane radios at an airport with a control tower (this is also illegal). As far as the pure mechanics of landing go, one does not need any instruments whatsoever: one can judge the airspeed by the sound and the wing angle. I routinely make my students do this when I teach tailwheel flying.
    - TheRealPomax 4y ago
      Not to mention that no one would ever be insane enough to turn off electronics and then say "right, now IFR your way in". You're flying VFR, you're going to have eyes on the runway.
    - stevehawk 4y ago
      > Not to doubt your story but being a flight instructor myself, I cannot imagine intentionally turning off power to airplane radios at an airport with a control tower (this is also illegal). You just pre-arrange it with the tower and it's no big deal. Probably exceptions for busy class Cs or Ds, but it's no big deal to any airport near me.
    - grepfru_it 4y ago
- V99 4y ago
  There are only 3 suffixes, so if there are 4 or more parallel runways one will get the "wrong" number, like 2/3L/3C/3R.

Jalad 4y ago

I agree with the other commenter, it's also explained in this CGP Grey video https://youtu.be/qD6bPNZRRbQ https://youtu.be/qD6bPNZRRbQ

mr_woozy 4y ago
[dead]

Rebelgecko 4y ago

So that they line up with the magnetic compasses inside of the planes

nilsbunger 4y ago

It's a convention for A LOT of aspects of flying: air traffic control instructions ("turn left heading 090"), charts ("fly heading 070 after takeoff"), etc are all in magnetic coordinates. Planes also a normal compass as their ultimate backup instrument, which of course is also magnetic. Finally, while modern systems could operate off true north instead, not ALL planes have that.

inamberclad 4y ago

Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Also, winds (and just about everything else) are in magnetic degrees. If I have wind from 280 while I'm landing, then I immediately know that I have a left quartering headwind and what to expect.

quest88 4y ago
Haha of course someone on HN would use 31 at PAO as their example :).
- fragmede 4y ago
  For the uninitiated, PAO is a small general aviation airport in the city of Palo Alto aka the heart of Silicon Valley. Nearby is also SQL, adjacent to Oracle's (former) HQ. That name is purely coincidence!
  - inamberclad 4y ago
    Hah, I never actually made the SQL <-> Oracle connection.
deepsun 4y ago
> Also, winds (and just about everything else) are in magnetic degrees. Not always: "If you read it, it's true. If you hear it, it's magnetic." https://aviation.stackexchange.com/a/62/51758 https://aviation.stackexchange.com/a/62/51758
shostack 4y ago
Having just flown out of there yesterday, on the return it is also helpful, it's also helpful to know without having to look it up what I should set my OBS to in order to help align for practicing my pattern and approach.
DocTomoe 4y ago
> Basic safety check. If I'm on RWY 31 at PAO, then my compass better read 310. Actually, it's more like "310ish". Both 305 and 314 are acceptable readings for a runway 31.

akira2501 4y ago

One other thing not covered, on any airport chart, the exact alignment of the runway will be noted, usually down a tenth of degree. So the numbering is a useful backup and discriminator, but the charts have much more detail, and modern navigational and instrument systems on runways obviate much of this for most flights anyways.

mmaunder 4y ago

It’s super helpful and intuitive when you’re flying. Took me a while to get the compass visualized in my head but now if I’m making a visual approach and they give me a runway I can look at my current magnetic heading on my heading indicator - which you periodically sync to your magnetic compass - and know exactly what direction relative to my current heading I’ll be landing. So that’s the fundamental reason why most things flight related are magnetic except rare items like winds aloft reports. Wind direction reports at the field? Magnetic, so that you know how many degrees off the wind is from your landing direction and can do a bit of mental math to get the crosswind component. If you’ve just taken off ATC may say fly runway heading - again an easy one using your heading indicator which is synced to your magnetic compass. As a side note, modern avionics like G1000 auto sync your heading indicator to magnetic. So if we started flying with electronic avionics that are auto synced and have a database of magnetic variation around the world, then it’s feasible to always show true on the heading indicator and have runways and everything else be true bearings. But that’s not the case and I still fly some of the many steam dial planes out there as do others, so we’re stuck with magnetic runways and magnetic everything else used in real-time flight and situational awareness for pilots.